This invention relates to a method for removing hydrogen sulfide(H2S) from a gas stream such as sour natural gas and acid gases, with the formation of elemental sulfur as a by-product.
It is well-known that numerous large untapped reserves of natural gas containing H2S currently exist, rendering such gas unsuitable for most uses. H2S-containing gas streams also originate in petroleum refineries and natural gas processing plants. Moreover, some natural gas reservoirs containing only a few percent of H2S have been kept shut-in because the high cost of removal of H2S and its conversion to sulfur exceeds the market value of the gas. Removal of H2S from such gas streams is necessitated by the high demand for such clean energy sources, by the value of sulfur as a by-product and by the need to regulate emmissions to meet air pollution standards.
It is known that H2S can be removed from a gas stream by absorption using an amine such as an alkanolamine. The two alkanolamines which are most commonly used are monoethanolamine and diethanolamine. After desorption of the H2S, the modified Claus process is typically used to convert H2S to sulfur. A furnace, two catalytic reactors and a clean-up unit may be operated in series to achieve sulfur recovery. However, this process is both capital and energy intensive. Further, the Claus process requires moderate to high concentrations of H2S, i.e above 30%, in the feed.
In U.S. Pat. No. 3,849,540, a process for removing H2S from natural gas is disclosed in which the H2S is removed by catalytic means. Specifically, the natural gas is treated with an aqueous solution containing dissolved oxygen and a transition metal catalyst.
In U.S. Pat. No. 3,761,569, a method for removing H2S from a gas stream is disclosed in which the H2S-containing gas is passed through a solution of cupric ions in a strong acid in the presence of oxygen. One of the acids disclosed is sulfuric acid, although the only examples are directed to the use of hydrochloric acid. This method fails if both oxygen and cupric ions are absent. It will also be appreciated by those skilled in the art that the presence of substantial amount of oxygen in such an environment is detrimental from both a corrosion and safety standpoint.
It is an object of the invention to provide a method for the removal of H2S from a gas stream, such as sour natural gas or H2S-containing acid gases.
It is another object of the invention to provide such a method in which elemental sulfur is preferentially formed.
Accordingly, a method for the removal of H2S from a gas stream is provided, comprising contacting the gas stream with sulfuric acid under conditions which preferentially favor the formation of elemental sulfur.
The method according to the invention is suitable for the removal of H2S from many gas streams, such as sour natural gas, petroleum refinery gas and inert gases, such as nitrogen gas, but is particularly well-suited to the removal of H2S from low-level H2S-containing natural gas streams i.e. those containing less than 5 mol % of H2S. However, the process has also been found to be applicable to gas streams containing concentrations of H2S of up to about 80%/w.
For example, for the production of elemental sulfur the following chemical reaction (1) is favored:
3H2S+H2SO4xe2x86x924S+4H2Oxe2x80x83xe2x80x83(1)
However, when flexibility in the product mix is of interest, the following chemical reaction (2) is favored:
H2SO4+H2Sxe2x86x92S+SO2+2H2Oxe2x80x83xe2x80x83(2)
That is, high conversion to elemental sulfur, accompanied by little or no conversion to SO2 is desired when sulfur production is paramount. To provide a feed for sulfuric acid production, complete conversion to SO2 would be optimal.
Applicants have found that higher concentrations of sulfuric acid and lower reaction temperatures favor formation of elemental sulfur, whereas lower concentrations of sulfuric acid and higher temperatures lead to increased formation of SO2. Accordingly, depending on the H2S content of the gas stream and the desired product mix, an appropriate balance between sulfuric acid concentration and reaction temperature is provided.
Defining SO2 selectivity as the percentage of H2S reacted to form SO2, for sulfur production, the treatment of sour natural gas would preferentially seek a 100% conversion of H2S and a 0% SO2 selectivity. To generate an SO2 feed for a sulfuric acid plant, 100% H2S conversion and 100% SO2 selectivity would be preferable.
More specifically, the applicants have found that by maintaining the reaction temperature between 22 and 125xc2x0 C. and the concentration of sulfuric acid between 80 and 96 weight per cent (wt %) (That is, between 80 and 96 wt % of concentrated sulfuric acid, the balance being water) the mix of products from reaction (2) may be varied in SO2 selectivity from 0 to 100%.
Preferably, the concentration of acid should be maintained between 82 and 96 wt %.
To further optimize the method, a reaction temperature of 20 to 150xc2x0 C., preferably 120 to 130xc2x0 C. and most preferably 125 to 127xc2x0 C. is provided.
The flow rate/contact time may also be controlled to further optimize the method. Flow rates of between 26 and 260 SCCM per 500 g of H2SO4 have been found useful, with rates nearer to the lower limit being preferred.